1. Field of the Invention
The present invention relates to speakers for use in various acoustic systems.
2. Description of the Related Art
A description of the structure of a conventional speaker will be given referring to FIG. 5. This speaker has magnetic circuit 1; a voice coil 4, of which at least coil section 3 is movably provided inside magnetic gap 2 of magnetic circuit 1; a diaphragm 5, the inner periphery of which being coupled to an external part of magnetic gap 2 of magnetic circuit 1; and a frame 7 to which the outer periphery of diaphragm 5 is coupled through edge 6. By inputting an electric signal fed from an audio amplifier and the like to coil section 3 of voice coil 4, voice coil 4 is excited, the force of excitation is transmitted to diaphragm 5, and diaphragm 5 converts the electric signal into a sound by vibrating air. An example of a speaker having such a structure is disclosed in Japanese Patent Unexamined Publication No. H11-275690.
In the above-described conventional example, as shown in FIG. 5, the inner periphery of damper 8 is fixed between coil section 3 of voice coil 4 and a section of voice coil 4 where the inner periphery of diaphragm 5 is fixed, and the outer periphery of damper 8 is fixed to frame 7. Damper 8 forms a suspension jointly with edge 6 and prevents voice coil 4 from rolling when in motion. Also, damper 8 is formed in the shape of two or more waveforms combined in order to minimize the mechanical load to voice coil 4.
However, according to a recent trend toward higher performance of speakers, the existence of damper 8 is causing serious problems.
That is, due to a large degree of non-linearity of mechanical load and asymmetry between the behavior of voice coil 4 moving toward magnetic circuit 1 and the behavior of the voice coil moving toward a direction opposite to magnetic circuit 1, there is a possibility of generating large harmonic distortion and, at the same time, worsening power linearity.
FIG. 6 shows power linearity of a conventional speaker, namely, amplitude (displacement) of diaphragm 5 as a function of input power to the speaker. In the Figure, curve A represents the amplitude characteristic of diaphragm 5 moving toward magnetic circuit 1 while curve B represents the amplitude characteristic of diaphragm 5 toward a direction opposite to magnetic circuit 1. Also, FIG. 7 shows the harmonic distortion characteristic of a conventional speaker. In the Figure, curve C, curve D, and curve E, respectively, represent the first harmonic distortion, second harmonic distortion characteristic, and third harmonic distortion characteristic of the speaker.
In order to solve such problems of power linearity and harmonic distortions, various studies are being made to improve non-linearity and asymmetry of damper 8. As has been described above, damper 8 is structured by combining two or more waveforms in order to minimize mechanical load. Accordingly, in so far as a suspension is to be structured by combining damper 8 and edge 6, it is difficult to reduce harmonic distortions by solving the problems of non-linearity and asymmetry, and enhancement of speaker performance is not satisfactory.
Therefore, a structure has been proposed in recent years in which damper 8 is removed and, instead, a ring-shaped suspension holder 8a is provided underneath diaphragm 5 as shown in FIG. 8 The inner periphery of suspension holder 8a is fixed to voice coil 4, and the outer periphery of suspension holder 8a is fixed to frame 7 through second edge 6a. Edge 6 and second edge 6a are made substantially symmetrical with respect to a space in between. That is, when edge 6 is upwardly protruding as shown in FIG. 8, second edge 6a is made to be downwardly protruding. With this structure, load unbalance in the vertical motion of diaphragm 5 associated with the shapes of the protrusion of both edges is cancelled, and worsening of power linearity is suppressed. This is an effort of making the two displacements of diaphragm 5 as shown by curves A and B in FIG. 6 identical.
However, there are problems to be solved with the speaker shown in FIG. 8. That is, as second edge 6a is provided, the magnetic circuit 1 has to be provided inward of second edge 6a. As a result, magnet 1a of magnetic circuit 1 becomes smaller, making the driving force of voice coil 4 smaller and presenting the possibility of smaller audio output.